A comparative analysis of the distribution of immunoreactive orexin A and B in the brains of nocturnal and diurnal rodents.

Nixon JP, Smale L - Behav Brain Funct (2007)

Bottom Line:
The orexins (hypocretins) are a family of peptides found primarily in neurons in the lateral hypothalamus.However, the present study shows significant species differences in the distribution of orexin cell bodies and in the density of orexin-IR fibers in some regions.Finally, we note previously undescribed populations of orexin-positive neurons outside the lateral hypothalamus in three of the four species examined.

Background: The orexins (hypocretins) are a family of peptides found primarily in neurons in the lateral hypothalamus. Although the orexinergic system is generally thought to be the same across species, the orexins are involved in behaviors which show considerable interspecific variability. There are few direct cross-species comparisons of the distributions of cells and fibers containing these peptides. Here, we addressed the possibility that there might be important species differences by systematically examining and directly comparing the distribution of orexinergic neurons and fibers within the forebrains of species with very different patterns of sleep-wake behavior.

Methods: We compared the distribution of orexin-immunoreactive cell bodies and fibers in two nocturnal species (the lab rat, Rattus norvegicus and the golden hamster, Mesocricetus auratus) and two diurnal species (the Nile grass rat, Arvicanthis niloticus and the degu, Octodon degus). For each species, tissue from the olfactory bulbs through the brainstem was processed for immunoreactivity for orexin A and orexin B (hypocretin-1 and -2). The distribution of orexin-positive cells was noted for each species. Orexin fiber distribution and density was recorded and analyzed using a principal components factor analysis to aid in evaluating potential species differences.

Results: Orexin-positive cells were observed in the lateral hypothalamic area of each species, though there were differences with respect to distribution within this region. In addition, cells positive for orexin A but not orexin B were observed in the paraventricular nucleus of the lab rat and grass rat, and in the supraoptic nucleus of the lab rat, grass rat and hamster. Although the overall distributions of orexin A and B fibers were similar in the four species, some striking differences were noted, especially in the lateral mammillary nucleus, ventromedial hypothalamic nucleus and flocculus.

Conclusion: The orexin cell and fiber distributions observed in this study were largely consistent with those described in previous studies. However, the present study shows significant species differences in the distribution of orexin cell bodies and in the density of orexin-IR fibers in some regions. Finally, we note previously undescribed populations of orexin-positive neurons outside the lateral hypothalamus in three of the four species examined.

Mentions:
Outside of the diencephalon, the distribution of orexin-IR fibers was more heterogeneous (see Table 2). Orexin-A and OXB fibers were scattered throughout all areas of the cortex; these fibers were more concentrated in inner and outer cortical layers, and slightly reduced in Layers 3 and 4 in all animals (Figure 15). The olfactory bulbs exhibited no fibers in any species examined (Fig. 16), but sparsely scattered fibers were present in the olfactory cortex and tubercle in all species. Orexin fiber density was uniformly low to moderate in the septal nuclei, claustrum, and amygdala, with the exception of the hamster medial amygdaloid nucleus, where orexin fibers were more dense than in the grass rat and degu, but not significantly more so than in the LE rat. The density of both OXA and OXB fibers was very low in the hippocampus and basal ganglia in all animals. The bed nucleus of the anterior commissure exhibited moderate to dense orexin fiber innervation in all species (Fig. 17).

Mentions:
Outside of the diencephalon, the distribution of orexin-IR fibers was more heterogeneous (see Table 2). Orexin-A and OXB fibers were scattered throughout all areas of the cortex; these fibers were more concentrated in inner and outer cortical layers, and slightly reduced in Layers 3 and 4 in all animals (Figure 15). The olfactory bulbs exhibited no fibers in any species examined (Fig. 16), but sparsely scattered fibers were present in the olfactory cortex and tubercle in all species. Orexin fiber density was uniformly low to moderate in the septal nuclei, claustrum, and amygdala, with the exception of the hamster medial amygdaloid nucleus, where orexin fibers were more dense than in the grass rat and degu, but not significantly more so than in the LE rat. The density of both OXA and OXB fibers was very low in the hippocampus and basal ganglia in all animals. The bed nucleus of the anterior commissure exhibited moderate to dense orexin fiber innervation in all species (Fig. 17).

Bottom Line:
The orexins (hypocretins) are a family of peptides found primarily in neurons in the lateral hypothalamus.However, the present study shows significant species differences in the distribution of orexin cell bodies and in the density of orexin-IR fibers in some regions.Finally, we note previously undescribed populations of orexin-positive neurons outside the lateral hypothalamus in three of the four species examined.

Background: The orexins (hypocretins) are a family of peptides found primarily in neurons in the lateral hypothalamus. Although the orexinergic system is generally thought to be the same across species, the orexins are involved in behaviors which show considerable interspecific variability. There are few direct cross-species comparisons of the distributions of cells and fibers containing these peptides. Here, we addressed the possibility that there might be important species differences by systematically examining and directly comparing the distribution of orexinergic neurons and fibers within the forebrains of species with very different patterns of sleep-wake behavior.

Methods: We compared the distribution of orexin-immunoreactive cell bodies and fibers in two nocturnal species (the lab rat, Rattus norvegicus and the golden hamster, Mesocricetus auratus) and two diurnal species (the Nile grass rat, Arvicanthis niloticus and the degu, Octodon degus). For each species, tissue from the olfactory bulbs through the brainstem was processed for immunoreactivity for orexin A and orexin B (hypocretin-1 and -2). The distribution of orexin-positive cells was noted for each species. Orexin fiber distribution and density was recorded and analyzed using a principal components factor analysis to aid in evaluating potential species differences.

Results: Orexin-positive cells were observed in the lateral hypothalamic area of each species, though there were differences with respect to distribution within this region. In addition, cells positive for orexin A but not orexin B were observed in the paraventricular nucleus of the lab rat and grass rat, and in the supraoptic nucleus of the lab rat, grass rat and hamster. Although the overall distributions of orexin A and B fibers were similar in the four species, some striking differences were noted, especially in the lateral mammillary nucleus, ventromedial hypothalamic nucleus and flocculus.

Conclusion: The orexin cell and fiber distributions observed in this study were largely consistent with those described in previous studies. However, the present study shows significant species differences in the distribution of orexin cell bodies and in the density of orexin-IR fibers in some regions. Finally, we note previously undescribed populations of orexin-positive neurons outside the lateral hypothalamus in three of the four species examined.